1. Field of the Invention
The present invention relates to a film lens and a surface light source using the same, and more particularly, to a surface light source adapted for back-lighting for a display unit, such as a liquid crystal display unit, illuminated advertising display, traffic-control sign, etc.
2. Information of the Related Art
FIG. 1 shows a conventional surface light source of the edge-light type, which comprises a light transmitting flat plate as a light guide plate 1, and is used as back-light source for a liquid crystal display unit (LCD). In this surface light source, a light beam is applied from both or one of the side end faces of the light guide plate 1 so that it is propagated throughout the plate 1 by utilizing total reflection in the light transmitting plate. Part of the propagated light beam is reflected by a light scattering reflector plate on the reverse side of the light guide plate 1, thus forming a diffused reflected light beam with an angle of reflection narrower than the critical angle, and the diffused light beam is emitted from the obverse side of the plate 1 (Jpn. UM Appln. Laid-Open publication No. 162201/1980).
In another example of the surface light source for back-lighting, as shown in FIGS. 2 and 3, a film lens 4, which has projections or lenticular lenses, each in the form of a triangular prism, on one side and a smooth surface on the other side, is stacked on the obverse side of a light guide plate 1 of the surface light source, with the projections upward. In this arrangement, a diffused reflected light beam can be uniformly diffused in an isotropic manner within a desired angular range by utilizing the light converging effect of the lens (Jpn. UM Appln. KOKAI Publications Nos. 4-107201 and 4-107237). When this film lens 4 is used in combination with a matte transparent diffuser sheet, the optical energy of the light source can be distributed more intensively within a desired limited angular range so that a diffused light beam with higher isotropy can be obtained in this range than in the case where a matte transparent diffuser sheet is used singly (U.S. Pat. No. 4,729,067).
In the aforementioned prior art arrangement (FIG. 1), however, the light scattering reflector plate is only provided on the reverse side of the light guide plate 1, so that the emitted light beam has a relatively sharp distribution with a peak angle of 60.degree. to a line normal to the obverse side of the light guide plate. Thus, the luminance with respect to the direction of the normal line, along which the brightest light is required, is insufficient, and the optical energy is dispersed in oblique directions in which less light is demanded. According to the alternative prior art arrangement (FIG. 2), the triangular lenticular film lenses on the light emitting surface of the light guide plate refractively converge the emitted light beam, so that the ratio of the optical energy of the light beam emitted within the angular range of 30.degree. to 60.degree. increases with its peak in the normal direction of the light emitting surface. As shown in FIG. 3, smooth surfaces on the reverse side of the film lens and the obverse side of the light guide plate are intimately in contact with each other and are integrated optically, so that total reflection cannot occur on the obverse side of the light guide plate.
As regards the luminance distribution within the plane of emission, therefore, high luminance is obtained within a distance of 2 to 4 cm from the source-side end portion of the light guide plate. The luminance gradually lowers with distance from the light source, and darkness is conspicuous in the region most remote from the light source (corresponding to the end portion on the opposite side to the light source or the central portion of the surface light source).
In order to eliminate these drawbacks, an attempt has been made to correct and equalize the luminance distribution within the plane of the light guide plate (Jpn. Pat. Appln. KOKAI Publication No. 1-245220). According to this arrangement, a light scattering layer on the reverse side of the light guide plate is formed in mesh patterns, and the area of the patterns is increased with distance from the light source.
In order to obviate the aforesaid drawbacks, moreover, another attempt has been made to correct and equalize the luminance distribution within the plane of the light guide plate (Jpn. Pat. Appln. KOKAI Publication No. 3-9306). In this case, two or more light sources are arranged around the side end portions of the light guide plate.
In either case, however, it is difficult to equalize the luminance perfectly. In the case of Jpn. Pat. Appln. KOKAI Publication No. 1-245220, the mesh patterns of the light scattering layer are inevitably conspicuous. In the case of Jpn. Pat. Apptn. KOKAI Publication No. 3-9306, on the other hand, the necessary space and power consumption of the light sources are doubled at the least.
As shown in FIG. 4, moreover, the diffusion angle may be controlled in two directions (vertical and horizontal) by combining two film lenses 4-1 and 4-2 in a manner such that their respective ridges extend at right angles to each other.
If the two film lenses 4-1 and 4-2 are stacked in layers, however, interference fringes of equal thickness (e.g., Newton's rings) are generated between unit lenses 42 on the lower film lens 4-1 and a smooth surface on the reverse side of the upper film lens 4-2, thereby lowering the image quality.